Secret carrier wave signaling system



July 7,1925. 1,545,270

H. W. NICHOLS SECRET CARRIER WAVE SIGNALING SYSTEM Filed No1 19,. 1920 To w21/whom# my concern.:

Patented July "7, 1925. f 'i uurll)Y STATES HAROLD W. NICHOLS, MAPLWOOD, NEWIJ'ER'SEY', ASSIGNOB vT0 'WESTERN IEIIEC4 TRIC COMPANY, INCORPORATED, OF NEW\ YORK, N. Y.; A CORPORATION Olie" NEW'l YORK.

. vsteam* CARI/@Inn wAvE sIefNALINGl SYSTEM.

y Application ivled Novcmber 19, 1920. Serial No. 425,075.

, Be it known um I, HAROLD w. NICHOLS, a citizen of the United` States, residing at 4vMaplewood, in the county of Essex, State of` New Jersey, have invented certain new and useful Improvements in Secret "Carrier Wave Signaling Systems, ofh which the fol` lowing is afull, clear, concise, and exact description. l,

' This? invention relates vto a signaling system in which signals are transmitted by the agency of a high frequency wave modulated in 'accordance'with signals,l and more particularly to a means for rendering the op:

eration of such a, system secret.

Its object is to provide -a system of wirev less or wire communication whereby secret vcommunlcatnm between statlons may be had .to the end that stations, other than the one' designed to receive, maynot receive intelligible signals..

The principle of the invention. resides in the transmission of an auxilila| wave hav# ing a particular frequency toget er with the modulated carrier wave. An attempt to de- -tect the received energy by any ordinary means results in the presence, in the output circuit'of the detecting'means, of a wave having'audible frequency and containing the elements of the signal wave but sufliciently distorted from the desired form as to bein-v capable of intelligent reception. l b

The expression secret signaling implies the provision of a means for normal-detection by the desired station and the invention accordinglyincludes as an additional feature y wave.'

lcircuit means at a receiving station'forren- I dermg. the auxlllary transmitted wave lneffective in interfering with the normally transmitted and detected signal wave.

More specifically the method of secret. signaling according to the presentv invention involves the following steps.: An inaudible frequency carrier wave is modulated in accordance with an audible frequency si al lT'he modulation is accomphshe by' some means, preferably a .balanced type l modulator, which produces aV pure moduf lated wave, the unmodulated component, orv

such component 'and one side band, being suppressed. 'A pure modulated wave may,

for purposes of deining Aapplicants inven- V tion, be described as any modulated carrier wave from which the unmodul-ated carrier component has been substantially eliminated.

, brieiydescri The output wave of the modulatoris trans l mittedtothe distantlstation together with an auxiliary wave. The frequencyv of the auxlliary wave is so selected as to dilerjrom that of the unmodulated component by au audiblel frequency and means are provided to insure that intermodulation does not Aoccur at the transmitter between this wave and the remaining transmitted components. At the recelving station,I the received waves are detected. The product of the detecting l frequencies' of the pure modulated wave transmitted. The resultingy component is v subjected to a, second detecting operation in which i-t is combined.l with a wave from a local source having a frequencyv equal to the difference between the frequencies of the unmodulated component and auxiliary waves at .the transmitting station. This, operation results in the production of a low frequency wave similar in allrespects to ,the signal wave and which may be separated from the 'remaining components resulting from the operation and caused to energize al telephone receiver to give -an acoustic sign j The princrleofthe invention has been d above. Before entering on a more detailed disclosure of the invention, v

especiallyas embodiedin a specic form,

attention will be given to the mode of opera` tion ofcertain elements which recur in several places in the system in slightly different forms. lIncluded among these elements are the balanced type modulator, the ,function of which has been brieliy described, and thev more common detector or single tube modu lator. i

' InI this connection reference is made to British `Patent No., 102,503, which contains a detailed account of the operation of a balanced'modulator, as well as a disclosure :of the basic theory of the detecting or modulating p operation., According .to the mathematical analysis therel set forth of the 1;)

action ofaa modulator,.and is equallytrueune obtained but these ar as applied to any two of the input waves of a detector, the complex wave resultlng from the combination of two input waves may be .resolved into component waves two of which lated component, and the upper andlower' side bands. The double frequency harmonies of each of the inputA waves are also e 1n general, widely different in frequency from the compenents to be utilized and are therefore readily separable therefrom.

The combination' of two modulator tubes in a' balanced modulator system according to the arrangement of the patent serves to suppress the unmodulated carrier component leaving onlyV the two high frequency components representing the side bands.

The detecting operation of a tube having a characteristic similar to that ofa modulator tube, yieldsa wave made up of components having frequencies of the sum and difference of the input frequencies. If an unmodulated carrier wave component is transmitted, or if the unmodulated carrier component issuppressed at the transmitter ,Y and reinserted in the'receiving conductor,

as in the so called homodyne system of the British patent, the combination in the d etector of the component frequencies according to the principles above enunciated results in a current of signal frequency which can be separated from currents of other frequencies and used to actuate a telephone receiver."

'For a more detailed disclosure of the in- 'vention,` as embodied in a practical form, reference is made to the following description taken in connection with the accompanying drawing in which Fig. 1 s hows in a diagrammatical form an arrangement of the transmitting circuits of the lnvention;

Fig. 2 shows an arrangement of the receiv- .ing circuits of the invention and Fig. 3 shows an alternative form of a portion of the circuits of Fig.v 1. v

Referring to Fig. l, M is a modulator of the balanced type which is fully disclosed "in the British Patent No. 102,503 above mentioned and'will not be further described here, except to point out that the high frequency unmodulated component 1s impressed on the common input circuit by means of source C, and that the low freuency signal component is impressed on t e input circuits of both modulator units by means of low frequency source S which maybe a microphone, as shown, although t, other means may be used depending on the particular use to be made of the system.

Although aUparticula-r' type of modulator has been shown, other types which have ining the side band frequencies c-s and G+S Y as has been described, the unmodulated component of carrier4 frequency c having been suppressed.

According to the present invention currents of the side band frequencies are transmitted together with currents Ahaving an auxiliary frequency c. This balanced network D conjugately. connecting'the auxiliary frequency sourceK with the modulator output, by means 4of which the side band and auxiliary frequency components are independently transmitted without intermodulation. The prevention of intermodulation is due both to the fact that the conjugate points .have no difference of potential for currents of frequency which are therefore not impressed on modulator M and to the non-modulating characteristic of amplifier A. The artiiicial network R functions in the usual manner to perfect a balance in the two portions of the divided network. The currents of frequencies 0 8, -c-l-s and c are independ- .ently transmitted by means of transformer F to.transmitting conductor E -which is here shown as a radio antennaalthough it can equally well be a terminus of a transmission line.

By means of the arrangement shown, an interceptor is unable, by any ordinary means, to intelligibly detect the received the. sum and difference frequencies 2c, 2,

c-i-c-i-s, a-i-c-B, c-c-s and k--c--s' are obtained as well as additional, inaudible, frequencies. The .component having frequency 2 is audible and,taken alone, would.

be intelligible as speech although having a dierent pitch from the transmitted speech. However, if frequency c-o is within audible limits, the component c-c-s will be is accomf plished by means of the hybrid'col or within the audible range andof a character to ei'ectually mask the speech frequency, 'inasmuch as this band ofv frequencies will correspond, depending upon the particular signal Waves if the frequency k is properly no M value vof s-c, to amore' or less complete frequency k-c is given. a value Asufiicientl-y high, although within audible limits, it is single source.

possible by means of a receiving .system constructed -accordin Ito the' present invention yto receive intelllgiblesi als;l

Referring now to Fig. 2, t ere is shown a circuit at a receiving station which en'- I ables a person familiar withthe/principles of the invention to receive'v intelligible signals. IThe three waves v 8, c--s and lc which are transmitted from 'transmitting conductor E are received'by'receiving conductor G, here shown as an antenna, and detected by detecting means D1. The product of the detecting operation, as has just been pointed out includes com onents ofv current having thesum and di erence fre` quencies lc-c-l-s, c-c-s and 2, these be- I* ing the only components having audible The currents of.- frequencies frequencies.

band ilter' BF c-c-i-s are transmitted by which suppresses frequencies above and below the limits of the frequency band determined by this expression.

Currents of this frequency are combinedl with currents of frequency c-c from source Kl-C by mea-ns of detector D2, the currents being impressed -on its input circuit by means `of transformers J and R1. The output cur` rent of Athis detector includes components and `.9.` If c-'c has a value corresponding substantially, to `theupper limit cfA speech frequencies,4the componentj-c-c-l-s vcan be easily separated from theI com onent fre-4 quency .9. Fiveor six 'thousan cycles has; been considered a good value forV this diff ferencefrequency and if given this valuefthe component k-c may also be separated .from

the 'speech frequency component, although the constantv toneresulting from this component would not. rendery the speech unintelligible. 'l l The yspeech 'frequency' components are separated from these other components by means of low pass filter LF and are' transmitted to the indicating means whichmay be a telephone receiver." Itmay be desir-y 'I able to am lify the voice frequency' currents and am li er A, and transformer Oare ac; cordingly shawn inter osed in circuit be? tween the filter and in icatin 'dev ic'e.

t is apparent from-the a ove-,that the, efficient operation lof the` s stem depends receiver equal to thedifference of frequenl cies k and c at the transmitter. Since this difference depends on the frequecies of two sources, it is subject to greater casual variation than is trueof source K-C. By the arrangement of Fig. 3, the frequencies c and cmay be so related that the difference therebetween may be more nearly constant than by the means shown in Fig. 1 and substantially as constant 'asif the current of this,

difference frequency were the product of a The circuits of Fig. 3 may be atpplied to 'p the system of Fig. 1. A portion othe output of high frequency c is combined with a current from'source T in modulating device M,. The resulting unmodularted component and'lower side band are suppressed by means of high pass filter HF which transmits' currents having a frequencymeasured by the sum of thefrequencies t and c. For a properly'selected value of t, which, forex# f ample, maybe given a value lz-e, the. difference frequencyl will b e the same as the frequency c from the source.,K of Fig. 1. If

the frequency c variesthis derived frequency will .no longer equal'k but it is readily apparent that it will increase with frequency c and that thediference between this frequency .and that of c willgbeaconstant. Ac-

cordingly, any fluctuation in the difference frequency c-c will b e duethe fluctuation in lthe source T alone and a more constant effect is secured'than by theV arrangement l'o disclosed in Fig. 1. v Althou h the system of the invention hasl been disc osed in connection with a radio transmission system, the theory of operation, is -not inconsistent with its use in a wiretransmission system and the invention 'is not tobe understood as limited-in scope by such disclosure, Ab'ut only by the scope of the appended 'claims. l i What is claimed is: 1. lhe method of, secret carrier Wave'sigf naling comprising, at the transmitting stalos tion,'sthe steps of' modulatinga' high fre- .i quency earrierwave in accordance with a signal wave, independently transmitting the modulated carrier wave and .an auxiliary wave differing in Afrequency from the carrier lfrequency by an audible amount, and at the receiving station, the steps of4 detecting said transmitted wave(i causing intermodulation between the pro uct cf said detecting 4operation and a 'wave having a frequency equal to the difference between the carrier and auxiliary wave frequencies, and separat# ing the signal frequency component from the i i resulting pro uc 2. A method of secret v,carrier wave signaling comprising the 4stepsofV modulating a high'frequen'cy carrier wave according toy a signa ,suppressing the unmodulated com largely on keeping the frequency .ke-0 at the, ponent, transmitting fthe remaining compo- '180 nents and transmitting therewith, but independently thereof', an auxiliary frequency wave having a frequency diffe-ring from the frequency of said carrier wave by an audible amount.

3. The method of carrier wave signaling A`which comprises the steps of generating a carrier wave, modulating said wave in accordance with control impulses, suppressing a portion of said modulated wave including substantially all of the unmodulated carrier component, radiating the resultant wave, and radiating simultaneously therewith but independently thereof an auxiliary 1- wave differing in frequency from the cartecting the said, transmitted waves, causing intermodulation between the product of said detecting operation and a wave having a 'frequency lequaly to, the difference between the carrier and auxiliary' frequencies, and separating the signal frequency component from the resulting product.

5. The method of secret carrier wave signaling which consists in modulating a highfrequency carrier wave in accordance with a wave'of a band of frequencies, suppressing the unmodulated carrier component, independentl-y transmitting at least one of the modulated carrier side band components and 4 an auxiliary wave 'differing in frequency from the, unmodulated carrier component by an audible amount, receiving and detecting said waves,'filtering out the ower side band com onentf of the wave resulting from the com ination in the detecting device'of the auxiliary frequency wave and the lower transmittedside band, modulating therewith a wave having a frequency equal to` the frequencies, filtering said waves, filtering out difference between the auxiliary' and carrier .out the signal frequency component o the modulating operation and causing said sial frequency component to actuate an in 'eating means.

6. The method of secret carrier telephony which consists in modulating a carrier wave in accordance with a signalv suppressing the unmodulated carrierA component, independently transmitting atleast a portion of the resultant wave and an auxiliary wave differing in frequency' from the unmodulated coml l,ponent by substantially the upper limit of speech frequency, receiving and detectin the lower side ban `component of the wave resulting from the 'v side band com onent resultin combination in the detecting device of |the auxiliary frequency wave and the lower transmitted side band, modulating therewith a wave having a frequency equal to the difference between the 'auxiliaryand carrier frequencies, filtering out the signaling frequency component of the modulating operation and causing said signal frequency component to actuate an indicating means.

7. The method of radio telephony of the modulated carrier current type which comprises the steps of modulating a carrier wavei in accordance with a signal wave, suppressing the unmodulated carrier component, independently radiating theJ resultant wave and an auxiliary wave having a frequency differing from the unmodulated component by substantially the upper limit of speech frequency receiving and detecting said waves, filtering out the lower side band component of thevwave ,resulting from the combination in the detecting device of the auxiliary frequency wave and the lower transmitted side band, modulating therewith a wave having a frequency equal tothe diff-erence between the auxiliary and the carrier frequencies, filtering Out the si al frequency lcomponent of the modulating operl ation and causing said signal frequency component to actuate an indicating means.

' 8. A secret carrier wave signaling system comprising in. combination means for enerating a carrier wave,means for modulating said wave in accordance with a signal, means for -suppressing the unmodulated carrier component, an auxiliary source adjusted to generate a wave having a freqbuency differing fromv the carrier frequency an audible amount, means for 'inde endent y transmitting the resultant modu ,ated wave and said auxiliary wave,.a receiving conductor, a detecting device, a local source. adjusted to gencrate a wave having a frequency equal to the difference between the frequencies of said carrier and auxiliary waves, ai modulating means 'for combining with the output 4wave of said detecting device the wave 4from said local source, and means for separating the signal frequency,I component from the out'-- put -wave ofsaid modulating means.v

9. A secret carrier wave radio telephone system comprising in combination a duplex balanced type modulator adjusted to produce a pure modulated wave, a source of carrier waves and a source of signal waves associated with said modulator, an auxiliary source adjusted. to generate a wave having a frequency differing from the carrier frequency by substantially the upper limit of speech, meansvfor independently radiating said pure modulated wave and said auxiliary wave, -a receiving conductor, a detector, a.

band filter adapted to transmit the lower combination in t e detectoi of t e auxiliary from thev y component of the modulator output wave,l

waves and the lower side band of the pure modulated wave, a local source adjusted tp generate a wave having a frequency equal to the'difference betwen the frequencies of said carrier and auxiliary'waves, a modulating means for combining said component 'with a wave having a frequency equal to the difference between frequencies of said for transmitting only the speech frequency and an indicating means.

`10. The method specified in claim 2 in v. which the auxiliary wave is derived from the carrier wave by a method comprising the steps of intermodulating a portion of the output of the carrier source with a wave having a frequency which differs from the desired auxiliary frequency by substantially the mean carrier frequency, and suppressing all but the desired auxiliary frequency component resulting from said modulation.

11..A method of secret carrier wave signaling which comprises the steps of generating a carrier wave, modulating said wave in 35 least one of the remaining side bands, and

transmitting therewith, but independently thereof, an auxiliary frequency wave having 90 a frequency differing from the frequency: of

lsaid carrier wave by an audible amount. carrier and auxillary waves, a low passilter 12. A. secret carrier wave signaling'system comprising means for genera/ting a carrier wave, means for modulating said wave inaccordance with a signal wave, means for suppressing the unmodulated carrier component, an auxiliary wave source adjusted to generate a wave differing in frequency from the carrier wave by an audible amount, andy means for independently transmitting at least a portion of the resultant modulated carrier wave and said auxiliary wave.`

In witness whereof\l hereunto subscribe my name this 11th day of November, A. D.,

HAROLD w. NICHOLS. 

